Multi-electrode gas sensors and methods of making and using them

ABSTRACT

A resistive gas sensor has a gas sensing layer (11) overlaid as a layer on an array of electrodes (12, 14, 16) with unequal gaps (22, 24) between them. Signals from the different electrodes represent resistances in different regions of the sensing layer. The layer is applied as a succession of sub-layers, with application of each sub-layer modifying the microstructure of the preceding sub-layers. The resulting variation in microstructure within the sensing layer (11) is used for distinguishing between a reactive gas and a less reactive gas. The sensor has domestic applications for carbon monoxide detection.

This invention relates to resistive gas sensors (also referred to asgas-sensitive resistors, or sensing devices), of the multi-electrodekind, i.e. a resistive gas sensor having three or more electrodes forreceiving signals from different regions of a gas sensing element of thedevice. The invention also relates to methods of making such sensors,and to methods of detecting a target gas using a multi-electroderesistive gas sensor.

Such sensors will also be referred to herein as multi-electrode arraydevices.

Multi-electrode resistive gas sensors are disclosed in, for example, thedocument W092/21018, which teaches operating principles ofmulti-electrode gas-sensitive resistors which are self-diagnostic. Theseprinciples are developed further in the document W095/04927. Referenceis invited to those documents for more information; and to the papers byD. E. Williams and K. F. E. Pratt, in J. Chem. Soc. Faraday Trans.,1995, 91, 1961 (referred to herein, for convenience, as "Williams I"),which presents the theoretical basis for the operation of the sensors,and in J. Chem. Soc. Faraday Trans., 1995, 91, 3307 (referred to herein,for convenience, as "Williams II"), which describes the experimentaldemonstration of the ability of such devices to detect poisoning oftheir own surfaces.

The gas sensing element consists generally of a porous body (typically athin layer) of an oxide, which is to be understood to includecombinations (such as a mixture) of more than one oxide, with or withoutadditives for various purposes. Such optional additives may includecatalytic material, for example to promote combustion of a particulargas in the mixture to which the sensor is exposed.

One of the objects of this invention is to provide a resistive gassensor capable of distinguishing between two gases (e.g. a reactive gasand a less reactive gas) in a gaseous mixture. One example of a reactivegas, in this context, is ethanol, and one example of a less reactive gasis carbon monoxide.

A further object of the invention is to provide a sensor which makes anoptimal distinction between a real hazard, such as CO, and a false alarmcaused by, for example, ethanol.

According to the invention in a first aspect, a resistive gas sensorincluding: a porous gas sensing element comprising an oxide as activegas-sensitive material, the sensing element having a working surface forcontact with an atmosphere; and at least three electrodes in electricalcontact with the sensing element, for receiving signals from differentregions of the latter, is characterised in that the microstructure ofthe sensing element is graded as between different regions of thesensing element.

Preferably, the said microstructure is finer in the basal region than inregions of the element closer to its working surface.

The sensing element is typically in the form of a layer, whichpreferably comprises a plurality of sub-layers, overlaid one on another,with each sub-layer having a different microstructure from the othersub-layer or sub-layers.

In preferred embodiments of the invention the electrodes comprise afirst electrode, a common second electrode defining a narrow gap betweenthe first and second electrodes, and a third electrode defining a widegap between the second and third electrodes, whereby output signals fromthe first electrode represent electrical resistance in a basal region ofthe sensing element close to the electrodes, and output signals from thethird electrode represent resistance across the whole thickness of thesensing element defined between the electrodes and the working surface.

Preferably, the active sensing material is chromium titanium oxide, withan impurity content comprising Cr₂ O₃ in the inclusive range 0-30 mol %and/or TiO₂.

It will be understood from this that such impurities may be entirelyabsent, though these and other impurities can be present, as isdiscussed later herein.

In some embodiments the sensing element includes up to 30% by weight ofcatalytically active material.

According to the invention in a second aspect, in a method of making asensor according to the invention, the sensing element is applied as alayer over the electrodes, and in that the said layer is applied insuccessive stages, each said stage comprising:

screen printing a sub-layer over the electrodes or a selected surfacearea of the electrodes, or over the last preceding sub-layer as the casemay be; and

drying the sub-layer,

whereby the application of each sub-layer other than the first tends tomodify the microstructure of the sub-layer or sub-layers previouslyapplied.

According to the invention in a third aspect, in a method of detecting atarget gas in a mixture of gases, using a resistive gas sensor having atleast three electrodes to produce electrical resistance signals, themethod including processing said signals to obtain information about thetarget gas and/or the mixture, the sensor is a sensor according to theinvention.

In this method, where the mixture includes a reactive first gas and aless reactive second gas, the sensor used has a sensing element theactive sensing material of which displays a concentration gradientacross the sensing element in response to the first gas, butsubstantially none in response to the second gas. Sensors according tothe invention are especially useful in this context, for distinguishingone gas from the other by the respective presence and absence of aconcentration gradient.

A particular example of a practical application of this method is forsensing carbon monoxide, whereby the sensor can distinguish the targetgas CO from less reactive gases, such as ethanol, the presence of whichcould otherwise cause false alarms to be given. An example of the use ofthe sensors of the invention for such a purpose is in domestic premises.

One preferred sensing material for sensors for use in the methods of theinvention is chromium titanate (chromium titanium oxide), in particularCr_(2-x) Ti_(x) O_(3+y), where 0.45≧x≧0.1 and y is a variable dependenton temperature and oxygen partial pressure, as discussed in the documentW095/00836, to which reference is invited for more detail. Chromiumtitanate is known per se as a material for gas-sensitive resistors; itis a preferred material for some purposes, particularly when prepared asa single-phase material with x=0.2 and operated at temperatures in therange 300-500° C. The Applicants have however been surprised to find inthis connection that the exact composition is not critical formanufacture of functional gas-sensitive resistors, although, foroptional performance, both purity (chemical and phase purity) andmicrostructure must be carefully controlled.

As regards impurities, the Applicants have found that functional sensorscan be prepared having a very wide range of elements present asimpurities, up to 1 atom %. Examples include Na, K, Ca, Mg, Pb, Cd, Bi,Si, Fe, Co, Ni, Ag, S and other alkaline, alkaline earth and transitionmetals, semi-metals and non-metals, Pt, Pd, Ir, Rh, Au and otherprecious metals. Such elements may indeed be added deliberately (as isknown in the art), variously, in order (a) to improve adhesion tosubstrate and electrodes, (b) to control sintering in the oxide layer,(c) to promote cohesion of the oxide, and/or (d) to modify theconcentration profile of the target gas or its decomposition products.

We have also found that phase-purity is not essential for satisfactoryperformance: unreacted Cr₂ O₃ present at up to 30 mol % does not preventthe material being functional. Moreover, we have found with somesurprise that amounts of TiO₂ present beyond the single phase boundarylimit of x≈0.45 do not prevent the material being functional.

We have found that the admixture of catalytically active materials suchas Pt or Pd, either decorating the surface of the gas sensing material,or admixed into the gas sensing material, supported on an insulator suchas Al₂ O₃ and co-printed with the gas sensing material, cause usefulvariations in the response of the resulting device. Up to at least 30%by weight of the catalytically active material may be admixed with thegas sensing material. The catalytically active material may thentypically consist of alumina powder platinised with 5% by weight ofplatinum.

The above mentioned variations arise because the catalytically activematerial causes a decomposition or transformation of the target gas,resulting in a concentration gradient of the target gas and itstransformation products within the porous sensor structure. Such aconcentration gradient can be detected and used for distinguishingdifferent target gases, and for self-validating multi-electrode sensors,for example those described in the document W092/21018.

In particular, Cr_(2-x) Ti_(x) O_(3+y), whether or not treated withcatalytically active metals, is an excellent gas sensing material bothfor carbon monoxide in air and for ammonia in air. Surprisingly, we havefound that, contrary to current thinking, chromium titanate untreatedwith a catalytically active metal does not catalyse the combustion ofeither of these gases, or at least not to any significant extent, in thetemperature range at which it may be operated as a gas sensor.Consequently, a concentration gradient of these gases is not establishedwithin the porous sensor structure.

However, a concentration gradient of solvents such as acetone, ethanoland methanol, which are common interferents to the target gases incircumstances encountered in practice, is indeed established. Hence amultiple-electrode sensing device of Cr_(2-x) Ti_(x) O_(3+y) canparticularly easily distinguish between the real threat from a targetgas (such as CO or NH₃) and an interference by traces of solventvapours. It is therefore particularly advantageous to use such devicesof this material, whether or not it contains a catalyst.

In a sensing device with multiple-electrodes, the response is determinedby a parameter K_(T) =kh² /D, where k is the pseudo first order rateconstant for decomposition of the gas within the porous sensorstructure, h is the layer thickness, and D is the diffusivity of the gasthrough the porous sensor body.

The parameter K_(T) depends on the temperature and the microstructure ofthe sensor. This dependence on microstructure arises through thevariations of D and k with particle size and packing density. Thediffusivity D depends on the porosity, constrictivity and tortuosity ofthe porous structure. Porosity here means the fraction of the layervolume which is occupied by gas; while constrictivity is a measure ofthe cross-sectional area of the gas paths through the porous solid; andtortuosity is a measure of their length.

Because the decomposition reaction may be catalysed on the surface ofthe sensor material (at the gas-solid interface within the pores of thedevice), the rate constant k depends on its internal surface area.Furthermore, the sensitivity of the device might, in principle, bealtered by altering the microstructure. This principle is in fact wellknown per se for conventional gas sensors of the simple type having onlyone pair of electrodes, because the effect of the gas on the electricalconductivity of the solid is known to be exerted at the gas-solidinterface. It is generally considered in this connection that theconsequent modification of conductivity is confined to a zone of somerestricted depth below the surface. Consequently, it is considered thatthe effect of the gas might be greatest at the contacts betweenparticles, that it might depend on whether these contacts were confinedto narrow points or were in the form of "necks" between grains, and thatit ought to depend on the grain size and whether the grains wereagglomerated into larger lumps.

In consequence of the known effects of microstructure on sensitivity,other work in the field is strongly directed towards manufacture ofdevices with ever smaller particle size, and with controlled but verysmall particle size.

It might be thought self-evident that devices with the smallest particlesize consistent with stability of the microstructure at the operatingtemperature should be the most sensitive, and therefore the mostdesirable. However, in the multiple-electrode sensing devices of theinvention, their actual sensitivity pattern is not predictable in thisway, because a change in the microstructure not only alters sensitivity,but also alters the parameter K_(T) that controls the relationshipbetween the different outputs of the device, in ways which are neitherintuitively obvious nor evident from the prior art, even to the personskilled in the art, because of the multiplicity of factors involved.

The invention provides a very simple means whereby the surprising resultis obtained that subtle alterations in microstructure can be made whichhave beneficial effects on the discrimination between gases that can beachieved with multi-electrode array devices. In particular, theinvention permits optimisation of the discrimination between the signaldue to a less reactive gas such as carbon monoxide and that due to amore reactive gas, such as ethanol or other common solvents. Thisoptimisation relies on achieving a microstructure and operatingtemperature at which:

(1) the ratio K_(T) for the more reactive gas (e.g. ethanol) is greaterthan unity, so that a concentration gradient is established for that gaswithin the device; this is not difficult because vapours such as ethanolare easily combusted on the surface of the device, even with a coarse,open microstructure such as may be achieved by inducing agglomeration ofparticles during the synthesis of the sensor material;

(2) with the same microstructure and operating temperature, K_(T) forthe less reactive gas (e.g. carbon monoxide) is substantially less thanunity, so that there is no concentration gradient of carbon monoxidethrough the layer; and

(3) the microstructure is graded, in such a way that the sensitivity toall gases is greater in the inner part of the layer than in the outerlayer. This is achieved in general by providing a finer microstructurein the inner part of the layer, i.e. the basal region of the sensingelement, distal from the working surface of the latter exposed to thegas, and proximal to the electrodes.

The resulting multiple-electrode sensor has excellent sensitivity forcarbon monoxide and excellent discrimination between real alarms due tocarbon monoxide, and false alarms due to the presence of solvent vapoursor ethanol.

Some embodiments of the invention will now be described and discussed,by way of example only and, where appropriate, with reference to theaccompanying drawings, in which:

FIG. 1 shows an electrode layout for a gas sensor of themultiple-electrothe multiple-electrode type having two electrode gaps;

FIG. 2 is an enlarged scrap view (not to scale), in cross section on theline II--II in FIG. 1;

FIG. 3 shows the configuration of the sensing layer as a set ofsub-layers;

FIG. 4 is a diagram which shows the variation of resistance ratio onexposure of a chromium titanium oxide device according to the inventionto carbon monoxide and ethanol at low concentrations in air;

FIG. 5 shows the variation in the resistance itself, of a tin oxidesensor according to the invention having wide and narrow electrode gaps,in response to the presence of low concentrations of carbon monoxide andethanol in air;

FIG. 6 shows the variation of resistance ratio for a tin oxide sensor onexposure to low concentrations of carbon monoxide and ethanol in air;

FIG. 7 is a graph of the response against time for a multi-electrodechromium titanate sensor in the presence of acetone;

FIG. 8 is a graph showing the variation with time of the ratio of theresponses on a wide gap and a narrow gap between electrodes of thesensor, for the same data, and on the same time base, as FIG. 7;

FIG. 9 is a graph similar to FIG. 7 for the same sensor in the presenceof ammonia; and

FIG. 10 is a graph showing the temperature dependence of the kineticparameter K_(T) for the response of the self-diagnostic sensor toacetone.

FIGS. 1 and 2 show a typical layout for a multi-electrode sensor,comprising a flat substrate 10 (about 2 mm square, for example), havinga front or sensing side seen in FIG. 1, and a back side. Threeelectrodes 12, 14, 16 are carried on the front side of the substrate,and an oxide sensing layer 11 (or sensing element) is overlaid on theelectrodes, in electrical contact with the latter through a base surface28 of the element 11. In FIG. 1, the sensing layer is omitted, so as toshow the electrodes, but its outline is indicated at 11 in phantomlines.

The outer or working surface 26 of the sensing element 11 is exposed, inuse, to an atmosphere containing a target gas or gases to be sensed(which term is to be interpreted broadly, to include detection andmeasurement with a view to the monitoring, identification, and/oranalysis, as defined, of the target gas or gases). A "narrow" gap 18, of20 μm in this example, is defined between the inner electrode 12 and themiddle or common electrode 14, which here have an interleaved, comb-likeconfiguration. A "wide" gap 20, i.e. a gap wider than the narrow gap 18,and being of 100 μm in this example, is defined between the middleelectrode 14 and the outer electrode 16. Electric leads 22, 23, 24 arespot welded to the electrodes 12, 14, 16 respectively, so that a signalcorresponding to the resistance R_(N) across the narrow gap 22 can betaken from the sensor via the leads 22 and 23, and a signalcorresponding to the resistance R_(w) across the wide gap 20 can betaken via the leads 23 and 24.

Further leads 28 are connected to a heater (not visible) on the back ofthe substrate 10.

With reference to FIG. 3, the oxide sensing element here consists of alayer which can be considered as comprising a number of sub-layers 11¹,112², 11³. . . 11^(n). The boundary regions between these sub-layers,where one sub-layer merges into the next, are indicated by broken linesin FIG. 3. The sub-layer 11¹ is adjacent to the electrodes; the regionconsisting of the sub-layer 11₁ and (to a decreasing extent going awayfrom the latter) adjacent sub-layers can be regarded as a "basal" regionof the element 11.

The arrangement whereby the narrow gap 22 in FIG. 1 is defined by aninterdigitated pattern of narrow strips, forming part of the electrodes12 and 14, tends to cause the electric current to flow only through thebasal region of the sensing element. The outer, or wide, electrode 16not only defines the wide gap 20, but is also not interdigitated.Instead, the electrode 16 is in the form of wide strips: thisarrangement tends to cause the electric current to flow uniformlythrough the whole thickness of the sensing layer 11.

EXAMPLE 1

A mixed powder of chromium trioxide and titanium dioxide is prepared byweighing commercially available powders (such as those available fromAldrich Chemical Company) in the ratio 0.9 mole Cr₂ O₃ : 0.2 mole TiO₂,and placing these powders into a cylindrical vessel, together withmilling media (zirconia, alumina and steatite have all provedsatisfactory) and sufficient solvent (water, acetone, ethanol, isopropylalcohol and methyl ethyl ketone have all proved satisfactory) to make ahighly fluid mixture. This mixture is then ball-milled for sufficienttime to achieve an intimate mixture of the oxide powders.

Following this step, the milling media are filtered from the suspension,the solvent is evaporated and the resultant dry powder is fired for1000° C. for 1 to 16 hours. Shorter firing times, or lower firingtemperatures, are found to result in some unreacted chromium trioxideand titanium dioxide being present. Where this is the case, theperformance of the resulting devices is not the best achievable, butfunctionality is generally unimpaired.

The fired powder is mixed in a triple-roll mill with a conventionalformulation of solvent and polymer for preparation into an ink suitablefor screen-printing. This is then screen-printed on to aluminasubstrates carrying, on the front or sensing side, an array ofelectrodes, which may for example be generally as shown in FIG. 1. Onthe back side, the substrate has a platinum resistance track used bothto heat the sensing element and to maintain it at constant temperature.The ratio of powder to polymer in the screen-printing ink is adjusted togive an open porosity of some 30-60% in the final sensing element. Thefinished sensor is typically as already described with reference toFIGS. 1 to 3.

The sensor layer is deposited over the electrodes in a succession ofprinting steps with intermediate drying. Each step lays down a layerhaving a dried thickness of 10 μm. The intermediate drying is carriedout under an infra-red lamp, or in an oven at approximately 110° C. Atotal layer of thickness 90 μm is deposited in this way, in a successionof nine printing and drying steps.

The Williams I paper mentioned earlier herein showed that the importantfabrication parameters were the ratios a/h and b/h, where a is half thewidth of the inter-electrode gap, b is half the electrode width, and his the oxide layer thickness. In the example shown in FIG. 1, for thenarrow gap 18, a/h=b/h≅0.1, while for the wide gap 20, a/h≅0.8 and b/his very large.

Microscopy has shown that, with material fabricated in this way, thefinal device, viewed from on top, had a very open microstructure inwhich the basic crystallite size was 0.1-1 μm, and in which there wereboth large agglomerates (up to 10 μm) and large open pores (1-10 μm).Table 1, below, shows the results of exposure of this device topropan-2-ol and to carbon monoxide at low concentrations in air. Theresistance rose markedly on both electrodes in both gases, whichillustrates the problem of interference between the desired signal (i.e.the response to carbon monoxide) and false alarm signals (i.e. theresponse to solvents and alcohol vapour).

However, Table 1 also shows that the variation of the resistance ratio,T_(W) /T_(N), discriminated between the two gases. For propan-2-ol, theratio increased when the gas was present, as may be expected from theWilliams I and W092/21018 documents. This means that propan-2-ol showeda concentration gradient through the layer, having a much lowerconcentration in the basal region of the layer probed by the narrow gap.Consequently the resistance response for the narrow gap was smaller andthe ratio R_(W) /R_(N) increased. In contrast, for carbon monoxide,R_(W) /R_(N) was most unexpectedly found to decrease.

                  TABLE 1    ______________________________________    Behaviour of sensors prepared according to Example 1                     Propan-2-ol                               Carbon monoxide            Air      (200 ppm) (400 ppm)    ______________________________________    Sensor resistance    at 390° C.:    wide gap  684kΩ                         5.23MΩ                                   1.76MΩ    narrow gap              180kΩ                         850kΩ                                   627kΩ    Sensor response    Rgas/Rair:    wide gap             7.64      2.57    narrow gap           4.72      3.48    Resistance ratio              3.80       6.15      2.81    R.sub.W /R.sub.N    ______________________________________

This means that carbon monoxide can be distinguished from propan-2-ol bysimple inspection of the ratio and its change. Similar results have beenobtained with ethanol, ethyl acetate and other common solvent vapours.

FIG. 4 shows the variation of resistance ratio R_(W) /R_(N) for severalsensors according to the invention, in response to exposure to carbonmonoxide and to ethanol vapour at low concentration in air. Theresistance ratio decreased in response to carbon monoxide and increasedin response to ethanol.

This effect is believed to arise because carbon monoxide does not have anotable concentration gradient through the thickness of the sensingelement, and the sensitivity of the oxide sensing material to gases isgreater in the basal region of the sensing element. It appears to theApplicants that the gradation of microstructure is achieved as aconsequence of the pressure exerted on the lower sub-layers as furtherlayers are printed on top of them, with this pressure serving somewhatto disaggregate agglomerates and to force finer material into the basallayers of the structure.

EXAMPLE 2

A chromium trioxide-titanium dioxide mixture is prepared by thefollowing steps:

(1) Chromium hydroxide is precipitated from a solution of chromiumnitrate (1.8 mole) in water by the addition of ammonium hydroxide, andremoved by filtration, being then washed with water but not dried.

(2) Hydrated titanium oxide is precipitated by the addition of titaniumiso propoxide (0.2 mole) to pure water, with rapid stirring, andseparated by filtration, being then washed with water but not dried.

(3) The powders are mixed and re-suspended in water by stirring in arotary evaporator flask immersed in an ultrasonic bath. The combinedaction of the ultrasound and stirring produces a mixed colloidaldispersion of the hydrated oxides. With continued stirring andsonication, the water is removed by vacuum. The resulting powder isremoved, dried and filtered to prepare the mixed oxide.

In contrast to the material prepared as in Example 1, material made inthis way has been found to have a basic crystallite size of less thanabout 0.1 μm. Sensors prepared by screen printing of this material, inthe same way as in Example 1, have been found to have significantlyincreased sensitivity to carbon monoxide (R in 400 ppm CO/R in air isabout 4.6, in contrast to values of about 3 for the material of Example1). However, Table 2, below, shows that the ratio R_(W) /R_(N)increases, surprisingly, for carbon monoxide as well as for propan-2-ol,if not to the same extent.

The explanation of this phenomenon is that the increased internalsurface area results in a sufficient increase in combustion rate, anddecrease in diffusivity, to cause a concentration gradient of carbonmonoxide to appear.

                  TABLE 2    ______________________________________    Resistance Ratio for Sensors of Example 2                  Propan-2-ol                            Carbon monoxide    Air           (200 ppm) (400 ppm)    ______________________________________    R.sub.W           3.8        6.2       4.7    R.sub.N    ______________________________________

EXAMPLE 3

A sensing device is prepared by printing the first 20 μm of thethickness in two layers, using material prepared as in Example 2. Thenext 70 μm of thickness is applied in seven layers, using materialprepared as in Example 1. The resulting device is found to have, on thenarrow electrode gap, an enhanced sensitivity to CO but not ethanol. Asa consequence:

(a) On exposure to ethanol, the signal is no bigger than that in Example1, and the ratio R_(W) /R_(N) increases as in Example 1.

(b) On exposure to carbon monoxide, the signal (using the narrow gap) isincreased above that obtained in Example 1, and approaches that obtainedin Example 2.

(c) The ratio R_(W) /R_(N) is found to decrease on exposure to carbonmonoxide, and this decrease is larger than with the sensor material ofExample 1. Therefore the device of the present Example has both animproved sensitivity to Co and an improved discrimination between CO andethanol.

EXAMPLE 4

A sensor comprising a porous layer of tin dioxide is prepared by mixinga commercially available tin dioxide powder (e.g. from the AldrichChemical Company) with a screen-printing medium. The sensing element isapplied by printing in a succession of layers with intermediate drying,following the general procedures of Example 1. The resulting devices arefired and tested. This has been found to produce a coarse, openmicrostructure, as for the material of Example 1.

FIG. 5 shows that the resistance of these devices has been found to fallon exposure to low concentrations of both carbon monoxide and ethanol inair, reflecting the fact that tin dioxide is an n-type material, incontrast to chromium titanium oxide, which is p-type. FIG. 6 shows thatthe two gases could again be distinguished from each other by observingthe behaviour of the resistance ratio R_(W) /R_(N).

The expected behaviour is now a fall in this ratio, corresponding to alarger signal on the wide electrode gap, and this has indeed been foundfor ethanol. Again surprisingly, the resistance ratio changed in theopposite direction for carbon monoxide, which the Applicants attributeto the gradation of microstructure caused, as mentioned earlier, by theprocess of printing the devices in successive layers with intermediatedrying.

EXAMPLE 5

In this example, it is demonstrated that a multiple-electrode gas sensorcan distinguish between a combustible gas and a non-combustible gas.

In this case, by way of non-limiting example only, these gases areacetone and ammonia respectively. The sensor device in this example ismodified from one described in the Williams II paper cited above, inthat it is 3 mm square and has three pairs of electrodes, with gaps of20, 40 and 200 μm. The signals from the small and medium gaps have beenfound to be virtually identical to each other: therefore only theresults from the wide and medium gaps are considered here. Theelectrodes used in this case were of screen printed, laser-trimmed gold,on an alumina substrate which carried a platinum heater track printed onthe obverse side. The gas sensing material was Cr_(2-x) Ti_(x) O_(3+y),where 0.45≧x≧0.1, y being as defined earlier herein.

This oxide was admixed with 25% w/w platinised (5%) alumina powder, andprinted to a thickness of 100 μm.

Experiments have been performed by applying either acetone (0.05-10 Nm²)in air or ammonia (0.8-50 Nm⁻²) in air to this self-diagnostic sensingdevice, at temperatures between 593° K. and 744° K. The device response,G=R_(gas) /R_(air))-1 (as to which, see the Williams I paper), wascalculated for each electrode pair.

Typical behaviour for acetone and ammonia is shown in FIGS. 7 and 9respectively. FIGS. 7 and 9 show the response G to acetone and ammonia,respectively, as a function of time for the wide (200 μm) gap, shown asa continuous line, and for the medium (40 μm) gap shown as a brokenline. The readings for both Figures were taken at a temperature of 664°K. for the following acetone or ammonia concentrations (all in Nm⁻²):

    ______________________________________               FIG. 7                     FIG. 9    ______________________________________    (1)          0       0    (2)          10      12.5    (3)          5       0.8    (4)          2.5     25    (5)          0.1     0.8    (6)          0.05    50    ______________________________________

It will be evident from FIGS. 7 and 9 that a concentration gradientexists for acetone, but not for ammonia. Therefore the two gases can bedistinguished and simultaneously detected, using, for example, themethods described in the Williams I paper or in the document W092/21018.

FIG. 8 shows the response of the medium electrode gap relative to thatof the wide gap, for the same acetone data. It can be seen that theconcentration gradient decreases with increasing gas concentration. Inthis connection, it should be noted that the Applicants have found thatthe combustion kinetics are liable to become oxygen limited, aphenomenon with which FIG. 8 is consistent. For simplicity, however, thekinetics are treated as pseudo first order in this case, and acorrection is made by extrapolation (see below). The concentrationgradient becomes steeper with increasing temperature, as expected.

Using the nomenclature described in Williams I, the response of thedevice towards acetone will now be analysed in terms of thedimensionless parameters K_(p), K_(T) and β.

As already mentioned, K_(T) =kh² /D, with k the first order rateconstant, h the layer thickness and D the gas diffusivity. The parameterK_(p) is given by Kp=A_(g) c.sup.β, where Ag is the responsecoefficient. From the data, the value of β, the order of the response,was found to be 0.6. This unusual value of β (values of 1, 1/2, 1/4 etc.are common) is thought to be due to morphological effects on theobserved response.

For the planar geometry used here, it was shown in the Williams I paperthat the dependence of normalised response on K_(T) was approximatelyindependent of K_(p), for K_(p) <10. Given this, the value of K_(T)could be determined independently of K_(p), simplifying the analysis andgreatly reducing the number of simulations required.

Numerical simulation was used to predict the ratio G_(S) /G_(L) as afunction of the parameter K_(T) for β=0.6 and K_(p) =1. This was thenused for the determination of the apparent value of K_(T) for each valueof concentration and temperature. A corrected value of K_(T) (in theabsence of oxygen-limited kinetics) at each temperature was thendetermined by extrapolation of the apparent K_(T) values to zero acetoneconcentration, since the kinetics would indeed become first order at lowgas concentrations. An Arrhenius dependence of K_(T) with temperaturewas observed (see FIG. 10). The value of the parameter k/D was thusfound to be 5.7×10⁹ e^(-5380/T) T^(1/2) cm⁻². This is several orders ofmagnitude higher that values determined by the Applicants foruncatalysed materials, as would be expected because of the platinumcatalyst admixed into the sensor material.

From the simulated data and calculated values of K_(T), the temperaturedependence of the response coefficient A_(g) was found to be 12.7_(e)⁻⁰.02r (Nm²)⁻⁰.6. The computed value of K_(p) varies from 0.1 to 5.2over the temperature and concentration range studied, justifying theapproximation K_(p) <10.

We claim:
 1. In a resistive gas sensor including: a porous gas sensingelement comprising an oxide as active gas-sensitive material, saidsensing element having a working surface for contact with an atmosphere;and at least three electrodes in electrical contact with said sensingelement, for receiving signals from different regions of said sensingelement, the improvement wherein said sensing element has a basal layerin electrical contact with said at least three electrodes, said basallayer being overlaid with a plurality of sub-layers, each sub-layerhaving a different microstructure from the other sub-layers or -layerwhereby the microstructure from the basal layer through the plurality ofsub-layers changes in coarseness from said basal layer to said workingsurface.
 2. A sensor according to claim 1, wherein said electrodescomprise a first electrode, a common second electrode defining a narrowgap between said first and second electrodes, and a third electrodedefining a wide gap between said second and third electrodes, wherebyoutput signals from said first electrode represent electrical resistancein a basal region of the sensing element close to said electrodes, andoutput signals from said third electrode represent resistance across thewhole thickness of said sensing element defined between said electrodesand said working surface.
 3. A sensor according to claim 2, wherein eachof said first and second electrodes includes a set of strip portions,interleaved with those of the other electrodes to define the narrow gapas a serpentine gap.
 4. A sensor according to claim 1, wherein theactive sensing material is chromium titanium oxide, with an impuritycontent comprising Cr₂ O₃ in the inclusive range 0-30 mol % and/or TiO₂.5. A sensor according to claim 4, wherein the active material isCr_(2-x) Ti_(x) O_(3+y), where 0.45≧x≧0.1, and y is a variable dependenton temperature and oxygen partial pressure.
 6. A sensor according toclaim 1, wherein said sensing element has an impurity content comprisingone or more elements in the inclusive range 0-1 atom %.
 7. A sensoraccording to claim 1, wherein said sensing element includes up to 30% byweight of catalytically active material.
 8. A sensor according to claim7, wherein said catalytically active material is dispersed on thesurface of said active material.
 9. A sensor according to claim 8wherein at least one said sub-layer includes said catalytically activematerial mixed with the active material of that sub-layer.
 10. A sensoraccording to claim 7, wherein said catalytically active material ismixed with said active material.
 11. A method of detecting a target gasin a mixture of gases, using a resistive gas sensor having at leastthree electrodes to produce electrical resistance signals, the methodincluding the steps of processing said signals to obtain informationabout the target gas and/or the mixture, wherein said resistive gassensor is a sensor according to claim
 1. 12. A method according to claim11 in which the mixture includes a reactive first gas and a lessreactive second gas, wherein the sensor used has a sensing element theactive sensing material of which displays a concentration gradientacross the sensing element in response to the first gas, butsubstantially none in response to the second gas.
 13. A method accordingto claim 12 in which the second gas is carbon monoxide, and wherein saidsensor is a sensor according to claim
 4. 14. A method of manufacting aresistive gas sensor wherein said sensor includes a porous gas sensingelement comprising an oxide as active gas-sensitive material, saidsensing element having: a working surface for contact with anatmosphere, at least three electrodes for receiving electrical signalsfrom different regions of said sensing element, a basal layer inelectrical contact with said at least three electrodes, said basal layerbeing overlaid with a plurality of sub-layers, each sub-layer having adifferent microstructure from the other sub-layers or -layer wherebysaid microstructure from said basal layer through the plurality ofsub-layers increases in coarseness from said basal layer to said workingsurface, said method comprising the steps of screen printing eachsub-layer over the electrodes or a selected surface area of theelectrodes, or over the last preceding sub-layer as the case may be, anddying each sub-layer before application of any further sub-layer.